function changeImg(){  
    var imgSrc = document.getElementById("imgObj");
    imgSrc.src = chgUrl(imgSrc.src);
}  
//时间戳  
//为了使每次生成图片不一致，即不让浏览器读缓存，所以需要加上时间戳  
function chgUrl(url){  
    var timestamp = (new Date()).valueOf();  
	url = url + "?timestamp=" + timestamp;  
    return url;  
}  
